Shadow artefacts when rendering with bevel-modification
It is possible that you have duplicate geometry, or that the corners are made up of 2 unconnected faces. To clean up your mesh, you can try Mesh &gt; Clean Up &gt; Merge by Distance. This will &quot;snap together&quot; all vertices that are within a certain distance of each other.
If this doesn't solve your problem, you might have to manually inspect the faulty areas of the mesh. Select the  vertex you are having issues with, and move it out of the way to see what's going on. This might help you understand how the mesh is connected, and what is causing the problems.